Current-based method and apparatus for detecting and classifying arcs
An apparatus and technique are provided for generating a plasma using a power supply circuit and arc detection arrangement. The power supply circuit has a cathode enclosed in a chamber, and is adapted to generate a power-related parameter. The arc detection arrangement is communicatively coupled to the power supply circuit and adapted to assess the severity of arcing in the chamber by comparing the power-related parameter to at least one threshold. According to various implementations, arc occurrences, arcing duration, intensity and/or energy are measured responsive to comparing the power-related parameter to the at least one threshold. According to further implementations, the above-mentioned measured quantities are accumulated and/or further processed. An apparatus and method are also provided for detecting arc events when the current spikes above a threshold level. The method and apparatus is also for classifying the arc events based on the voltage and current signals and the duration each is beyond a threshold value.
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The present invention is a continuation-in-part of U.S. patent application Ser. No. 10/769,023, filed Jan. 30, 2004, which is a continuation of U.S. patent application Ser. No. 10/121,445, filed Apr. 12, 2002. The present invention also claims the benefit of priority of U.S. Provisional Patent Application No. 60/783,346 for “Current-Based Method and Apparatus for Detecting and Classifying Arcs” filed Mar. 17, 2006.
TECHNICAL FIELDThe invention generally relates to a method and apparatus for detecting and classifying arcs in a device, and more particularly, to a method and apparatus for detecting and classifying arcs in a physical vapor deposition process by examining the current and voltage, and timing the duration each goes beyond one or more threshold values.
BACKGROUND OF THE INVENTIONSputtering deposition, such as Physical Vapor Deposition (PVD), is a process for depositing thin, highly uniform layers of a variety of materials onto many objects, for example depositing a metal layer over a substrate such as a wafer used in forming integrated circuits (ICs). In a direct current (DC) sputtering process, the material to be deposited (target) and the substrate to accept the deposited material (wafer) are placed in a special vacuum chamber. The vacuum chamber is evacuated and subsequently filled with an inert gas, such as argon, at low pressure.
The wafer is electrically connected to (or in the vicinity of) the anode of a high voltage power supply, the anode being generally at or near earth potential. The walls of the sputtering chamber are also placed at this potential. A target, typically formed of metal, is placed in the vacuum chamber and electrically connected to the cathode of the high voltage power supply. Alternately, the target is formed of an insulating material. An electric field is generated between the target (cathode) and an anode by the power supply. When a potential between the anode and cathode reaches 200-400 volts, a glow discharge is established in the inert gas in the superconducting region of the well known Paschen curve.
When a glow discharge operates in the superconducting region of the Paschen curve, valence electrons are torn from the gas and flow toward the anode (ground), while the resulting positively-charged ionized gas atoms (i.e., plasma) are accelerated across the potential of the electric field and impact the cathode (target) with sufficient energy to cause molecules of the target material to be physically separated from the target, or “sputtered.” The ejected atoms travel virtually unimpeded through the low pressure gas and plasma, some of which land on the substrate and form a coating of target material on the substrate. The result, under ideal conditions, is a uniform cloud of target molecules in the chamber, leaving a resultant deposition of uniform thickness on the chamber and its contents (e.g., the wafer). This coating is generally isotropic, conforming to the shape of the objects in the chamber. A natural consequence of this action is that the target material wears or becomes thinner as more material is sputtered.
The processing of integrated circuits is reliant on the uniformity of coating resulting from the glow discharge process. The vacuum chamber containing the discharge and target material is carefully designed to attempt to maintain a uniform electric field, and a glow discharge is, in principle, sustainable over a range of electric field strengths, again in accordance with the Paschen curve. However, uniformity of electric field cannot be maintained perfectly and the uniformity of the glow discharge and henceforth wear on the target is influenced by a number of factors, including thermal currents generated in the chamber and other mechanical anomalies, such as target misalignment. To compensate for these anomalies, commercial PVD sputtering machines often incorporate a mechanism to rotate a large magnet at constant speed above the target. This rotation serves to disturb the electromagnetic field in the chamber, focusing the region in which the plasma impinges upon the target on a smaller, moving area. Maintaining a constant power in the chamber while rotating the magnet at a constant rate improves the uniformity of wear of the target, increasing target life and generally maintaining a more uniform distribution of molecular target material in the chamber. As the magnet rotates above the target, local geometric, thermal and other variations cause the lumped electrical impedance of the chamber to change. With the power supply configured to deliver a constant power to the glow discharge, the relation between chamber voltage and current required to maintain constant power changes in accordance with the variation in impedance. If one monitors the chamber voltage and current, a clear periodic variation in the chamber voltage and current can be observed, with the period equal to that of the rotational period of the magnet.
Even with the rotating magnet mechanism in place to attempt to stabilize the glow discharge, certain conditions can result in a local concentration of the electric field causing the glow discharge to pass from the superconducting region of the Paschen curve into the arcing region. Arcing during PVD results in an unintended low impedance path from the anode to the target through electrons or ions in the plasma, the unintended path generally including ground, with the arcing being caused by factors such as contamination (i.e., inclusions) of the target material, inclusions within the structure (e.g., surface) of the target, improper target alignment (e.g., misalignment of cathode and anode), vacuum leaks, and/or contamination from other sources such as vacuum grease. Target contaminants include SiO2 or Al2O3.
Arcing during PVD is one cause of yield-reducing defects in forming integrated circuits on semiconductor wafers. While normal metal deposition is typically less than 1 micron thick, arcing causes a locally thicker deposition of metal on the wafer. When an arc occurs, the energy of the electromagnetic field of the chamber is focused on a smaller region of the target than intended (e.g., the neighborhood of the target defect), which can dislodge a solid piece of the target. The dislodged solid piece of target material may be large relative to the thickness of the uniform coating expected on the wafer, and if a large piece falls upon the wafer, it may cause a defect in the integrated circuit being formed at that location. Subsequent photolithography processing etches away various areas of the deposited metal layer, leaving metal conductor paths according to desired circuit patterns. Because arcing results in a localized defect (area) having a greater thickness than the surrounding metal, the defect area may not be thoroughly etched in the subsequent processing, resulting in an unintended circuit path (i.e., short) on the chip. A semiconductor chip has multiple metal layers separated by insulator layers, each of the metal levels formed by depositing, patterning and etching a metal layer as described above. A local defect in one layer can also distort an overlying pattern imaged onto the wafer in a subsequent photolithography step, and thus result in a defect in an overlying layer.
Manufacturing a wafer of modern integrated circuits can involve well over a thousand individual processing steps, the value of the wafer and consequently each individual integrated circuit die increasing with each processing step. Arcing in a PVD sputtering apparatus used to process wafers into integrated circuits can render portions of the wafer useless for its intended purpose, thereby increasing manufacturing costs. Using target materials free of arc-causing inclusions is one way of minimizing integrated circuit fabrication defects; however, target material may become contaminated during its manufacture or thereafter. Discovering target contamination prior to sputtering operations so as to prevent arcing defects is costly, both in terms of time and expense. Not discovering arcing defects in a timely manner is similarly costly in terms of random yield loss, for example by the manufacturer operating a deposition chamber until the target inclusion causing the arcing is sputtered through. Furthermore, when a solid piece of the target is dislodged during an arc, the surface of the target may be further damaged and the potential for future arcing in that neighborhood increases.
Absent real-time arc detection, corrective action is dependent upon the availability of parametric data. It is costly to measure the number of defective layers caused by arcing, for example via electrical tests designed to reveal shorts or by scanning the surface of wafers with a laser after metal deposition. These tests take time to run, during which production is delayed, or undetected yield loss occurs for an extended time. Since a defect such as a short at any level can impact integrated circuit functionality, it is desirable to avoid damage resulting from arcing during sputtering deposition.
Accordingly, real-time arc detection permits faster identification of sources of yield loss, and detection of incipient faults within the processing tool or target itself, both resulting in more efficient integrated circuit fabrication applications.
As discussed above, arcs can throw solid material into the chamber, and it can be assumed that any such piece of solid material landing on a wafer of integrated circuits has a high probability of damaging at least one integrated circuit. One statistic indicative of the potential damage to a wafer of integrated circuits is therefore the number of arcs that occur during a process step. It is also reasonable to assume that the expected damage caused by an individual arc to an integrated circuit wafer is a monotonically increasing function of the energy delivered to the arc, since a violent arc is likely to spread more solid material over a wider area than a relatively “mild” arc. A system that can estimate both the number of arcs occurring during a PVD sputtering process step as well as the severity of the arcs in real time is therefore a valuable tool in estimating the potential damage caused in a particular PVD sputtering step.
It is well known that when an arc occurs in a glow discharge process, the magnitude of the lumped impedance of the chamber decreases rapidly. When this occurs, the presence of series inductance in the driving point impedance of the power delivery system, comprising power supply and interconnection means, causes a rapid drop in the magnitude of observed voltage between the anode and cathode of the chamber. Observing the chamber voltage and comparing it against a fixed threshold is a common means of detecting the presence of an arc and one can readily accomplish this by attaching a common oscilloscope to the cathode, with the ground of the oscilloscope probe attached to the chamber. Having an estimate of the average chamber processing voltage, which one can obtain visually by observing the voltage using a free running oscilloscope, one can set the trigger point of the oscilloscope at a voltage greater than the expected voltage (the voltages observed in such a manner are negative with respect to the oscilloscope reference). When the oscilloscope triggers, the resulting voltage waveform due to the arc can be observed and one can also simultaneously observe the current by means of an appropriate current probe. Systems have been developed that emulate this method of detecting arcs and which count the number of occurrences so obtained over the course of a processing step. A known shortcoming of this approach is that the fixed trigger level must be set conservatively, as the chamber voltage varies periodically with magnet rotation as discussed above, as well as varying over the course of a PVD processing step due to thermal and other considerations. As such, such a system may miss arcs of small magnitude, which nonetheless cause damage. A system that can more closely follow the actual, instantaneous expected chamber voltage would permit these arcs to be detected more readily, providing a more accurate estimate of damage.
In the PVD process used to produce integrated circuits, arcing conditions lasting less than 1 microsecond are commonly observed. These short duration arcs are commonly called microarcs. Electronically controlled analog or switching power supplies cannot react to this rapid change in chamber impedance during a microarc. As a natural consequence of the series inductance, the power supply delivers a near constant current to the chamber during a microarc. Assuming that during an arcing condition, all energy delivered by the power supply is focused on the arc, the energy delivered to an individual arc can be estimated by the integral of the product of the power supply voltage times the (assumed constant) current over the interval of the arc. Again, digital oscilloscopes exist that permit the capture of both the chamber voltage and current waveforms during an arcing condition. Computer software, such as Tektronix “Wavestar” software, exists that can permit a digitally stored waveform to be uploaded to a computer, where the captured voltage and current waveforms can be subsequently multiplied point by point to compute instantaneous power and that power waveform integrated over the duration of the arc to determine the overall energy delivered by an arc.
While useful for gaining an understanding of the arcing phenomenon in PVD applications, this method of computing arcs and arc energy using an oscilloscope and a post processing computer is of little value in production applications. Even modern handheld oscilloscopes are relatively bulky instruments, and real estate in an integrated circuit clean room is extremely valuable. A stand alone post processing computer also takes up valuable floor space and would likely need to be located outside the clean room and connected to the oscilloscope by a network, adding latency in the transfer of data between the oscilloscope and computer. Furthermore, there is no means to tell a-priori the duration of an individual arc, or the frequency at which they might occur, leaving the problem of exactly how to set the controls of the oscilloscope. Oscilloscopes also have limited waveform storage capability, and therefore prone to losing information at the times in which it is needed most, when there is much arcing activity during a process. A system so configured would render real time control and decision making impractical.
Various aspects of the present invention address the above-mentioned deficiencies and also provide for arc detection methods and arrangements that are useful for other applications as well.
In addition to the problems discussed, when counting arcs only as voltage threshold violations, some information may be lost or obscured if the power supply responds to the arcs by reducing delivered power. The result of reducing power is a dip in both the voltage and the current.
The present invention is provided to solve the problems discussed above and other problems, and to provide advantages and aspects not provided by prior methods or systems of this type. A full discussion of the features and advantages of the present invention is deferred to the following detailed description, which proceeds with reference to the accompanying drawings.
SUMMARY OF THE INVENTIONAccording to one aspect of the invention, the present invention is directed to an apparatus and method for detecting arcs during plasma generation that addresses the above-mentioned challenges and that provides a feedback method for controlling film deposition processes. The present invention is exemplified in a number of implementations and applications, some of which are summarized below.
According to one example embodiment of the present invention, a plasma generation apparatus includes an arc detection arrangement communicatively coupled to a power supply circuit. The power supply circuit has a cathode enclosed in a chamber, and is adapted to generate a power-related parameter. The arc detection arrangement is adapted to assess the severity of arcing in the chamber by comparing the power-related parameter to at least one threshold.
According to other aspects of the present invention, the arc detection arrangement is adapted to estimate arc intensity, arc duration and/or arc energy.
According to another example embodiment of the present invention, the arc detection arrangement is implemented using a programmable logic controller (PLC).
According to another example embodiment of the present invention, the PLC operates in concert with the arc detection arrangement to compute an adaptive arc threshold value responsive to normal variations in the impedance of the PVD chamber, said real time adaptive arc threshold value communicated by the PLC to the arc detection apparatus in near real time.
According to another example embodiment of the present invention, the adaptive arc threshold value responsive to normal variations in the impedance of the PVD chamber is computed by the arc detection arrangement itself, with statistical data regarding both arcing activity and the adaptive arc threshold function communicated to the PLC in near real time.
Actual micro-arcs (e.g., as captured on an oscilloscope) show a rapid decrease (followed by a recovery to a nominal value) in voltage magnitude and simultaneously, a rapid increase (also followed by a recovery to a nominal value) in current magnitude. Accordingly, looking at the current level for spikes, and looking at the voltage level for simultaneous decreases greatly increase the confidence level or success rate of “true” arc detection. The present invention provides embodiments of a method and apparatus for detecting such arc events, and for detecting and classifying other arc events.
According to another aspect of the present invention, the output of a current transducer is fed into a programmable threshold comparator of an arc detection unit. In this embodiment, arc events are measured by the arc detection unit in terms of how many times current makes an excursion above a threshold value and in terms of the elapsed time for which the current is above the threshold value. Additional information regarding the severity of the arc may be obtained by placing more than one threshold value (each at a different level) above the nominal operating point and comparing arc event counts and elapsed time for the different threshold levels.
According to another aspect of the invention, the apparatus includes logic to classify arc events based on combined data from both the voltage and current channels of a power supply interface. Additionally, the apparatus computes scan energy and arc energy for events occurring in a particular class of arc events.
According to an example embodiment of the present invention a method of detecting and classifying arcs in a physical vapor deposition process is provided. The method comprises monitoring a power supply voltage and current of a plasma generation apparatus. Based on the monitoring the method includes detecting each instance when the voltage drops below a predetermined first voltage threshold, timing the duration of each instance the voltage drops below the predetermined first voltage threshold, detecting each instance the current spikes above a predetermined first current threshold, and timing the duration of each instance the current spikes above the predetermined first current threshold. The duration of voltage drops and the duration of current spikes can be measured in clock cycles. The method then comprises classifying each instance the voltage drops below the predetermined first threshold and each instance the current spikes above the predetermined first current threshold as an arc event. Accordingly, arc events can occur from either a detected voltage drop and/or a current spike.
The method further includes determining whether the power supply voltage is in one of a stable mode, a rising transition mode or a falling transition mode. Arc events may be counted or otherwise analyzed separately for each of these categories. For example, the method can include maintaining a count of arc events and corresponding durations occurring when the voltage is in a stable mode, maintaining a count of arc events and corresponding durations occurring when the voltage is in a rising transition mode; and, maintaining a count of arc events and corresponding durations occurring when the voltage is in a falling transition mode.
The arc events can be put into different classifications based on the data acquired from monitoring the power supply voltage and current of the plasma generation apparatus. According to one example, during a predetermined time period, such as a scanning cycle of a PLC or other computing device or logic arrangement or circuitry, the method includes assigning arc event instances where a voltage drop and a current spike are coincidental a first classification. Additionally, the method further includes assigning arc event instances of one or more voltage drops without a corresponding coincidental current spike having a cumulative duration less than a predetermined time a second classification and assigning arc event instances of one or more voltage drops without a corresponding coincidental current spike having a cumulative duration greater than a predetermined time a third classification. With respect to sensed current arc events, the method similarly includes assigning arc event instances of one or more current spikes without a corresponding coincidental voltage drop having a cumulative duration less than a predetermined time a fourth classification, and, assigning arc event instances of one or more current spikes without a corresponding coincidental voltage drop having a cumulative duration greater than a predetermined time a fifth classification. For each of the various classifications, the method can include calculating the scan energy for the designated arc events
Detecting an arc event often results in the power supply dropping (i.e., entering into a falling transition mode). To avoid including or counting transients resulting while in this falling transition mode as being in a stable mode, the method further includes disabling detecting a voltage drop below a predetermined first threshold for a transition hold period after each detection of a voltage drop below the predetermined first threshold, and disabling detecting a current spike above a predetermined first threshold for a transition hold period after each detection of a current spike above the predetermined first threshold. The information can still be kept if further analysis is done for the transition modes.
The method also accommodates the slow changes (i.e., relative to arc events) to the supply voltage occurring during a sputtering deposition process in the stable mode. In this regard, the method further includes adjusting the predetermined first voltage threshold during a scanning cycle to track slow changes in the supply voltage.
According to one example, the method could be set up to provide additional information regarding the severity of arcing. In this regard, the method can include detecting each arc event instance where the voltage drops below a predetermined second voltage threshold, and detecting each arc event instance the current spikes above a predetermined second current threshold. Additional threshold values can be similarly be utilized to provide even more precise information.
According to another example of the invention, a method of determining an arc event in a plasma generation apparatus comprises the steps of monitoring a power supply current, obtaining a current signal indicative of the monitored current, and, determining if the current signal is beyond a predetermined current threshold value indicative of an arc event. Similarly, the method can further comprise monitoring a voltage of the power supply. obtaining a voltage signal indicative of the monitored voltage, and, determining if the voltage signal is beyond a predetermined voltage threshold value indicative of an arc event. Additionally, for the method can include timing the duration of each arc event occurring when the current is beyond the predetermined current threshold value and when the voltage is beyond the predetermined voltage threshold value. Again, each arc event can be classified, and the scan energy and arc energy can be calculated.
According to yet another example of the invention a method for detecting arcs in a plasma generation apparatus comprises providing a supply of power to the plasma generation apparatus to create an ionized gas between a target and a wafer, providing an interface for detecting a supply voltage and a supply current, comparing the voltage to a voltage threshold value at a set frequency and, comparing the current to a current threshold value at the set frequency. Additionally, the method comprises determining if an arc event occurred from the comparing of the voltage to the voltage threshold value and from the comparing of the current to the current threshold value.
The method further includes delaying comparing the voltage to the voltage threshold value and the current to the current threshold value for a transition delay period after each detection of an arc event. This provides a more accurate arc event count for the stable mode.
Additionally, the method can include looking at other parameters (than voltage or current threshold crossings) to provide further information of any arcing. This can include further information regarding the severity of the arc events. According to one example, the method can further include the steps of generating a power-related parameter, comparing the power-related parameter to at least one threshold to determine the severity of arcing in the plasma generation apparatus and, measuring arc duration responsive to comparing the power-related parameter to the at least one threshold.
According to yet another aspect of the invention, an apparatus for detecting an arc event in a plasma generation chamber is provided. The apparatus comprises a power supply interface module configured for detecting a power supply voltage and current applied to the plasma generation chamber and, an arc detection unit communicatively coupled to the power supply interface module, the arc detection unit including a threshold comparator circuit arranged to compare the voltage to a first voltage threshold value for determining if an arc event occurs and compare the current to a first current threshold value for determining if an arc event occurs. The arc detection unit preferably includes a digital signal processor (DSP) with an analog to digital converter.
Additionally, the arc detection unit of the apparatus includes or is coupled to logic circuitry arranged to make a determination of an arc event based on an output of the threshold comparator circuit. The logic circuitry can be a programmable logic controller (PLC) or other similar computing device. Moreover, in some instances, the DSP could include logic to perform some or all of the functions disclosed herein.
The threshold comparator circuit is preferably programmable to enable a user to set an initial voltage threshold value and an initial current threshold value. Additionally, separate components can be used for the voltage and the current. The threshold comparator circuit is preferably an analog circuit. The threshold level is generated in the DSP and the arc signal is converted to digital by an analog to digital converter in the DSP. The DSP contains firmware whose parameters are software controlled by the PLC or other logic circuitry or arrangement.
The logic circuitry of the apparatus is utilized for a number of functions. For example, the logic circuitry is arranged to determined if the voltage is in one of a stable mode, a rising transition mode and a falling transition mode. Additionally, the logic circuitry is arranged to maintain a count of arc events occurring when the voltage is in a stable mode, maintain a count of arc events when the voltage is in a rising transition mode, and maintain a count of arc events when the voltage is in a falling transition mode. The logic circuitry is also arranged to determine the duration of an arc event based on the voltage dropping below the first voltage threshold value, and the duration of an arc event based on the current spiking above the first current threshold value. The duration is typically measured in clock cycles which—based on the frequency—can be converted to time units.
The logic circuitry is further arranged to classify arc events based on the output of the threshold comparator circuit and the duration of each arc event. The classification is for a predetermined time cycle, such as a PLC scan cycle. The logic circuitry can be configured, for example, to assign arc event instances where a voltage drop and current spike are coincidental a first classification, assign arc event instances of one or more voltage drops without a corresponding coincidental current spike having a cumulative duration less than a first predetermined time period a second classification, assign arc event instances of one or more voltage drops without a corresponding coincidental current spike having a duration greater than the first predetermined time period a third classification, assign arc event instance of one or more current spikes without a corresponding coincidental voltage drop having a cumulative duration less than a second predetermined time period a fourth classification, and assign arc event instance of one or more current spikes without a corresponding coincidental voltage drop having a cumulative duration greater than the second predetermined time period a fifth classification.
The logic circuitry can also compute various parameters of the arcing. This can include scan energy and arc energy.
According to a further aspect of the invention, an apparatus for detecting arcs in a plasma generation apparatus comprises an arc detection unit communicatively coupled to a current of a power supply. The arc detection unit includes a threshold comparator circuit configured to compare the current to a first current threshold value, and logic circuitry arranged to detect an arc event based on a comparison of the current to the current threshold value in the threshold comparator circuit.
The arc detection unit can also be communicatively coupled to a voltage of the power supply. In this instance, the threshold comparator circuit is further configured to compare the voltage to a first voltage threshold value and the logic circuitry is further arranged to detect an arc event based on a comparison of the voltage to the voltage threshold value in the threshold comparator circuit.
The arc detection unit further comprises a timing circuit arranged to compute a duration of a detected arc event based on the comparison of the current and the current threshold value. The timing circuit is also arranged to compute a duration of a detected arc event based on the comparison of the voltage and the voltage threshold value.
The threshold comparator circuit can be configured to compare the current to a second current threshold value (or a plurality of additional threshold levels) different than the first current threshold value. The threshold comparator circuit can similarly be configured to compare the voltage to one or more additional threshold values. The duration the current or voltage is beyond a particular threshold value can be computed for each of the threshold values.
According to yet a further aspect of the invention, an apparatus for detecting an arc event in a plasma generation apparatus comprises a power supply interface module communicatively coupled to a voltage and current of a power supply for the plasma generation apparatus, an arc detection unit having a first channel for receiving a signal indicative of the voltage and a second channel for receiving a signal indicative of the current; and, and a threshold comparator circuit in the arc detection unit arranged to compare the voltage signal to a voltage threshold value and to compare the current signal to a current threshold value.
The apparatus can further comprise logic circuitry to determine if an arc event occurred based on the output of the threshold comparator circuit. The logic circuitry can also be arranged to compute a parameter-related to the power supplied to the plasma generation apparatus. The logic circuitry can also compare the power related parameter to at least one threshold to determine the severity of arcing in the plasma generation apparatus.
The present invention enhances the ability to determine in real-time when arcing is occurring so that corrective action may be taken. This can improve wafer yield and reduce defects.
In some instances, an apparatus looking primarily at the voltage and power, would count both the arc and the resultant voltage dip (i.e., resulting from a reduced power supply response to the arc), which would produce an inaccurate count. The present invention further provides a method and apparatus for more accurately counting and classifying arcs. That is, by counting arcs as current threshold violations, even in the presence of power reduction events, the arcs are more accurately represented in arc counts and time statistics.
Other features and advantages of the invention will be apparent from the following specification taken in conjunction with the following drawings.
To understand the present invention, it will now be described by way of example, with reference to the accompanying drawings in which:
While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.
DETAILED DESCRIPTIONWhile this invention is susceptible of embodiments in many different forms, there is shown in the drawings and will herein be described in detail preferred embodiments of the invention with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the broad aspect of the invention to the embodiments illustrated.
The present invention is believed to be applicable to a variety of different types of plasma generation applications, and has been found to be particularly useful for film deposition applications, the latter benefiting from a technique for responding to detected arcs during the generation of a plasma environment. Example embodiments described herein involve PVD sputtering techniques; however, the present invention can be implemented in connection with a variety of systems, including those using plasma-generating techniques such as plasma etching or Plasma Enhanced Chemical Vapor Deposition system (PECVD).
While arcing events may never be wholly avoided, obtaining certain detailed data regarding the severity of arcs occurring during a sputtering process provides useful information from which compensatory-process decisions can be made. For instance, through real-time detection of a single arc of small magnitude, one might suspect the presence of minimal defects due to arcing on an affected integrated circuit die. Conversely, from real-time detection of a large quantity of arcs, or arcs of high severity, one might suspect the presence of many defects, perhaps even reach a conclusion that an entire processing step is defective. Real time arc detection according to the present invention permits manufacturing decisions to occur in real time, or nearly so. For example, where a processing step is suspected as being defective due to detection of significant quantity or severity of arcing, the PVD process step may be terminated before further damage can occur. At the end of a PVD processing step, whether completed normally or terminated per above a decision to repair or discard the wafer can be made before further processing steps are initiated. If an initial processing step is deemed defective through real-time detection of significant arcing, and processing costs to the present stage of manufacturing the wafer are low, it may be cost-effective to discard the wafer. If arcing occurs during a latter processing step, for which the cost of processing a wafer to the affected step is high, it may be cost-effective to chemically etch or physically polish the wafer to remove the defective deposition layer and re-process the wafer. Additionally, detection of arcing activity on a wafer to wafer basis of an individual PVD system in which no or minimal previous arcing activity is observed may be indicative of the development of an incipient equipment fault condition that can be corrected by scheduling appropriate equipment maintenance during scheduled equipment inactivity. The key is timely recognition of the increased probability of defects due to arcing.
For a particular PVD system, the power supply to drive the process attempts to regulate power delivered to the chamber. The impedance of the chamber elements, including the anode, cathode and chamber environment between the anode and cathode, is in series with the impedance of the plasma-generating power supply circuit. The relation between voltage and current to maintain a constant power in a plasma is dependent upon the impedance of the chamber elements, including the conductivity of the particular target material itself which is subject to change as a result of the sputtering process.
When an arc develops in the sputtering chamber, the magnitude of the impedance of the chamber drops rapidly, thereby changing the impedance of the plasma-generating power supply circuit. The power supply and distribution circuit contains significant series inductance, limiting the rate at which current can change in the circuit. A rapid drop in chamber impedance therefore causes a rapid decrease in the magnitude of chamber voltage due to this inductive component. This collapse in chamber voltage magnitude is often sufficient to extinguish the arcing condition and re-establish a glow discharge before serious damage to the chamber, the power supply, or the target can result. Typically, arcing events occur (and disappear) more quickly than the electronics regulating the power supply are able to react, so even if corrective action is initiated by the electronics, some damage to the wafer is possible. As discussed previously, the probability that an item being coated will suffer some form of defect, such as a non-uniform coating on a wafer, increases as a result of each arcing event. Because the chamber voltage drops rapidly when an arcing event occurs, an unexpected voltage drop below a pre-defined or adaptive voltage threshold level can be used to define the occurrence of an arcing condition.
The voltage threshold delineating the existence of an arcing event is dependent upon the nominally applied (i.e., non-arcing), perhaps time varying chamber voltage according to one example implementation. The non-arcing chamber voltage applied to produce a glow discharge is dependent on many factors including the condition and composition of the target (which affects circuit impedance). All other circuit impedances remaining constant, a higher chamber voltage is required to produce a glow discharge using a relatively less-conductive target material, conversely a lower chamber voltage is required to produce a glow discharge using a relatively-higher conductive target material. For example in one sputtering chamber implementation, the chamber voltage required to uniformly deposit aluminum is nearly twice the chamber voltage required to deposit copper. The chamber voltage required to uniformly deposit aluminum can also vary from chamber to chamber, being dependent on the balance of circuit impedance including the power supply and other chamber elements. Furthermore, as the target ages and more material is sputtered, the power required to maintain a uniform deposition rate must be modified (e.g., increased). As the required applied voltage changes, it follows that the associated threshold voltage at which an arcing condition is determined should also be changed.
According to a general example embodiment of the present invention, a plasma generation apparatus includes an arc detection arrangement communicatively coupled to a power supply circuit. The power supply circuit has a cathode enclosed in a chamber, and the power supply circuit is adapted to generate a power-related parameter (e.g., a voltage signal). The arc detection arrangement is adapted to assess the severity of arcing in the chamber by comparing the power-related parameters to at least one threshold. Parameters determining arcing severity are process-dependent and include, but are not limited to, arc quantity, arc rate, arc intensity, arc duration, and/or arc energy.
According to one implementation, the arc detection arrangement for a sputtering process monitors a sputtering chamber voltage and detects an arcing condition whenever the chamber voltage magnitude drops below a preset arc voltage threshold value.
According to one aspect, the power-related parameter (e.g., voltage) threshold value is variable over a range of power-related parameter values. Any threshold may be programmable, and may be controlled by a logic arrangement, for example being electronically controlled by a remote logic arrangement. In one example implementation, the voltage threshold value demarking an arc occurrence is computed in response to an estimate of nominal chamber voltage magnitude, the nominal chamber voltage magnitude being the chamber voltage necessary to produce a glow discharge (i.e., generate a plasma) during non-arcing conditions. In one example implementation, any threshold may be hysteretic, or programmed to be hysteretic having a “reset” value different from a “surpass” value.
According to one aspect of the present invention, the arc detection arrangement is further adapted to count arcing conditions (events) responsive to the at least one threshold. A rate of detected arcing condition occurrences may be determined therefrom.
According to another aspect, the arc detection arrangement is further adapted to measure arcing duration responsive to comparing the power-related parameter to the at least one threshold. For example, the arc detection arrangement includes a clock and a digital counting arrangement in one implementation. The clock provides a clock signal having a fixed period, and the digital counting arrangement is adapted to count the clock signal periods responsive to comparing the power-related parameter to at least one threshold. According to a further aspect of the present invention, the duration of arcing conditions is assessed by comparing the power-related parameter to at least one threshold. According to one example implementation, the duration of arcing conditions is accumulated over a fixed period. According to another example implementation, the duration of arcing conditions is accumulated until the duration threshold is reached, or until the accumulated duration is reset.
According to another aspect, the arc detection arrangement is further adapted to measure arcing intensity responsive to comparing the power-related parameter to the at least one threshold. In one example implementation, the arc detection arrangement is adapted to compare the power-related parameter to a plurality of thresholds arranged at different values thereby ascertaining the extent or range of change (from nominal) to the power-related parameter during an arcing event. In one example embodiment, the threshold corresponding to the largest observed voltage magnitude drop provides a lower bound to the energy estimate, while the next larger voltage drop threshold (which the system is observed to not exceed) provides an upper bound to the energy estimate.
According to another example embodiment of the present invention, the arc detection arrangement is adapted to measure arcing duration and intensity responsive to comparing the power-related parameter to the at least one threshold. In one implementation, the arc detection arrangement is further adapted to measure arcing energy responsive to comparing the power-related parameter to the at least one threshold, arcing energy being proportional to the product of the arcing duration and the arcing intensity, and assessment of arcing severity being a function of the arcing energy (i.e., the product of arcing intensity and arcing duration). According to one particular implementation, a plurality of thresholds are used to determine a plurality of durations, in order to estimate (i.e., approximate, or integrate) an area bounded by a power-related parameter (e.g., chamber voltage) versus time plot during a depression in the voltage due to arcing. An arcing energy proportional to the bounded area for each arcing event is used to assess the severity of arcing. According to a further implementation, the arc detection arrangement is further adapted to accumulate arcing energy over a plurality of arcing events, for example by summing the products of arcing intensity and arcing duration to assess the severity of arcing.
According to another example implementation, the arc detection arrangement includes a power-related parameter band-limiting filter as a means to prevent aliasing prior to digitizing the power-related parameter. Commonly understood digital signal processing techniques are applied to this digitized power-related parameter to reduce or accentuate certain frequency response characteristics of the power-related parameter. This digitally signal processed parameter may then be compared directly against a similarly digitized version of the at least one threshold.
According to another example implementation, a digitally signal processed parameter per above is used to compute at least one time varying threshold value, responsive to certain observed characteristics of one or more power-related parameter over the course of the PVD process.
According to another example embodiment of the present invention, a plurality of power-related parameters are compared to a plurality of thresholds in assessing the severity of arcing as described above. For example, in addition to chamber voltage, power supply current is monitored and used in detecting arcing events, an arcing event being determined whenever the current magnitude exceeds a preset current threshold value.
According to another example embodiment of the present invention, a logic arrangement is communicatively coupled to the arc detection arrangement, and adapted to process the arcing data collected by the arc detection arrangement. In one implementation, the logic arrangement is adapted to interface with the arc detection arrangement, the logic arrangement having a data network and additional external devices such as process controllers, monitors and logic arrangements. In one particular application, the logic arrangement is a programmable logic controller (PLC).
According to another example embodiment of the present invention, arc severity in a plasma generation chamber is assessed by timing an arc duration, which is derived by comparing a power-related parameter to at least one arc intensity threshold, and adding the arc duration to an accumulated arcing duration. Further example implementations of the method include measuring the power-related parameter during non-arcing plasma generation and automatically adjusting the arc intensity threshold(s) responsive to measuring the power-related parameter; counting arc occurrences; and/or assessing arc severity as a function of arc intensity, arc duration and/or the product thereof.
According to another embodiment of the present invention, arcing severity in a plasma generation chamber is assessed by determining an arc intensity, which is derived by: comparing a power-related parameter to at least one arc intensity threshold, timing an arc duration responsive to comparing a power-related parameter to at least one arc intensity threshold, computing arc energy as a function of arc intensity and arc duration, and then adding the arc energy to an accumulated arcing energy. Further example implementations of the method include measuring the power-related parameter during non-arcing plasma generation and automatically adjusting the at least one arc intensity threshold responsive to measuring the power-related parameter; counting arc occurrences responsive to comparing the power-related parameter to the at least one arc intensity threshold; and/or employing a hysteretic arc intensity threshold; and/or transmitting information representative of arcing to a logic arrangement on command via a shared data path, the information being one selected from a group that includes quantity of arc occurrences and accumulated arcing duration. The power-related parameter is a function of plasma generation chamber voltage in one particular implementation; the power-related parameter being formed as a digital representation of plasma generation chamber's operating characteristics in another implementation.
In describing the following particular example embodiments of the present invention, reference will be made herein to
An electric field is generated between the target (cathode) and anode by the power supply causing the gas in the vacuum chamber to ionize. Ionized gas atoms (i.e., plasma) are accelerated across the potential of the electric field and impact the target at high speed, causing molecules of the target material to be physically separated from the target, or “sputtered.” The ejected molecules travel virtually unimpeded through the low pressure gas and plasma striking the substrate and forming a coating of target material on the substrate. Typical target voltage for sputtering aluminum is a steady state magnitude of approximately 450 volts dc (VDC).
Transducer 220 has a first output terminal 222 carrying current signal I− and a second output terminal 224 carrying current signal I+. First and second transducer output terminals are electrically coupled to an Isense circuit arrangement 240, first transducer output terminal 222 being coupled to Isense circuit first input terminal 242, and second transducer output terminal 224 being coupled to Isense circuit second input terminal 244. Isense circuit arrangement 240 also has a first output terminal 246 carrying signal IPSIM−, and a second output terminal 248 carrying signal IPSIM+. Isense circuit receives current signals I+ and I−, and generates a differential voltage between signals IPSIM+ and IPSIM− responsive to the current flowing from the chamber to the power supply.
Vsense circuit 250 measures the potential difference between center conductor 215 and outer conductor 210, and generates a differential responsive to the potential difference. The Vsense circuit includes a first input terminal 252 coupled to inner conductor 215 and carrying voltage signal V−. The Vsense circuit also includes a second input terminal 254 coupled to outer conductor 210 and carrying voltage signal V+. The Vsense circuit has a first output terminal 256 carrying output voltage signal VPSIM−, and a second output terminal 258 carrying output voltage signal VPSIM+.
Coaxial cable 35, connecting power supply 30 to vacuum chamber 10, is terminated in standard commercial UHF type connectors in one example implementation. According to one aspect of the present invention, the mechanical packaging of PSIM 40 is arranged and configured such that cable 35 can be de-terminated at one end, inserted through aperture 225 of PSIM 40 and re-terminated to complete a circuit between power supply 30 and chamber 10. In an alternate implementation, PSIM 40 includes UHF type connectors so that PSIM 40 can be inserted in the circuit of cable 35 between power supply 30 and chamber 10.
According to the example implementation illustrated, resistor networks R3 and R4 provide an attenuation factor of 500:1 to each respective input voltage signal with respect to a reference plane, GNDANALOG. Each of the resistor networks R3 and R4 have a nominal resistance of approximately 20 Megaohms between the network sense terminal (pin 1) and the reference plane (pin 3). Resistive networks R3 and R4 can be implemented using, for example, thick film high voltage divider networks such as Ohmcraft P/N CN-470. An applied voltage of 1000 volts between 252 (V+) and 254 (V−) causes a current of 25 microamperes to flow into pin 1 of R4 and out of pin 1 of R3. Pin 3 of each of these voltage attenuators (i.e., resistive network) is coupled to the reference plane, GNDANALOG. Since each of the voltage attenuators provide a 500:1 attenuation, a differential voltage measured between pins 2 of each resistive network (i.e., between attenuated signal VPSA+ at pin 2 of R4 and attenuated signal VPSA− at pin 2 of R3) are attenuated by 500:1, and this measurement is independent of the voltage difference between either V+ and GNDANALOG, or V− and GNDANALOG.
The PVD sputtering chamber 10 has radio frequency (RF) energy applied in one example implementation, to stabilize the plasma. Capacitors C2, C3 and C5 of Vsense circuit 250 significantly attenuates (i.e., filters) this high frequency “noise”. According to one example implementation, the combination of C2 and R3 has an effective pole at about 22 kHz.
As discussed above, the differential voltage appearing between VPSA− and VPSA+ is a band limited representation of the signal appearing between V− and V+, with a nominal DC attenuation factor of 500:1. The equivalent DC Thevenin source impedance between VPSA− and VPSA+ is high (on the order of 80 kOhms) and therefore not suitable for transmission over large distances or into low impedance loads. Therefore, a differential instrumentation operational amplifier U2; for example an LT1920 instrumentation operational amplifier, is incorporated in the Vsense circuit to serve as a low impedance voltage follower. Operational Amplifier U2 provides high impedance inputs (pins 2 and 3), which will not significantly load the outputs of attenuators R3 and R4. Pin 2 of resistive network R3 is coupled to the inverting input (pin 2) of U2 and Pin 2 of resistive network R4 is coupled to the non-inverting input (pin 3) of U2. Resistor RG2 sets the voltage gain of U2 and is selected to yield a gain of 1 V/V in the example embodiment. The resulting output of U2 (pin 6) is a single ended low impedance voltage source relative to GNDANALOG that closely follows the voltage developed between VPSA− and VPSA+.
The output of U2 (pin 6) is coupled to the center terminal of a BNC type connector, J2, and carries the signal VPSIM+ 258. The outer connector of BNC type connector J2 carries signal VPSIM− 256 and is coupled to the reference plane GNDANALOG. The resulting differential voltage between signals VPSIM+ and VPSIM− is band limited with respect to the differential input signals V+ and V−, and has a nominal DC response of 2 mV/V.
In one embodiment, the Hall effect type DC current transducer 220 when coupled to an appropriate load impedance placed between signals 244 (I+) and 242 (I−) generates a current responsive to current flowing in inner power supply conductor 215. In one particular embodiment, using a model LA25-P Hall effect type DC current transducer manufactured by LEM, the current signal developed by DC current transducer 220 is approximately proportional to the total current passing through aperture 220 at a ratio of 1000:1. Thus a 1 ampere signal passing through aperture 220 generates a constant current of 1 mA flowing through an impedance placed between 244 (I+) and 242 (I−), within the limits of the DC current transducer design.
Power supply module U1 has three output terminals, +Vo, −Vo and Com. A +15 VDC signal is provided at terminal +Vo and a −15 VDC signal is provided at terminal −Vo. Vo. Terminal Com is coupled to the reference plane GNDANALOG. Pin 2 of connector J1 is also coupled to GNDANALOG as a common potential in the application as required. Resistors R1 and R2 and light-emitting diode D1 are coupled in series between the +15 VDC bias voltage and the −15 VDC bias voltage to provide an indication that PSIM power supply circuit 500 is operational.
Arcing is defined as a collapse in the chamber voltage magnitude that crosses a threshold voltage. Upon occurrence of an arc, the chamber (target) voltage magnitude rapidly decreases (i.e., is closer to ground potential), and chamber current increases more slowly due to series inductance, from steady state (i.e., non-arcing) conditions. The programmed threshold voltage is a predetermined chamber voltage at or below which an arcing state is determined and may be a constant value or a time varying function of the nominal, expected, possibly time varying chamber voltage. A non-arcing state is determined to occur when the chamber voltage is above the threshold voltage. According to an alternate example implementation, the threshold voltage is determined from a period including a non-arcing state, and an arcing state is defined to occur whenever the chamber voltage is below the voltage threshold. Multiple threshold voltages can be used to determine the magnitude of an arc (i.e., voltage dip or “severity”). For example, an arc that crosses a −200V threshold but not a −100V threshold may be considered less severe than an arc that crosses both thresholds.
The ADU 50 includes a digital signal processor to processes the signals received from the PSIM to provide digitally-filtered representations (e.g., digital signals), of the chamber voltage and current signals respectively, to a logic arrangement. According to one example implementation, the ADU includes an analog-to-digital converter (A/D).
The ADU is further adapted to set at least one programmable arc threshold voltage. In a further implementation, the ADU is also adapted to set at least one hysteresis threshold voltage. According to one aspect, the respective thresholds can be set at any point along a continuous spectrum; this can be affected via a potentiometer setting controlling a comparator circuit arrangement. According to another example implementation, the respective thresholds are set digitally via a digital to analog converter, or via a plurality of discrete threshold levels achieved by switching specific circuit components into a comparator circuit arrangement, for example by selecting the configuration of a resistive network. To identify the hysteresis threshold(s), the ADU provides a programmable hysteresis function to detect arcs that manifest themselves slowly. Both the arc (voltage) threshold and hysteresis function can be set or programmed directly in the ADU, or the threshold values may be optionally controlled by a remote device communicatively coupled to the ADU, for example through a standard Momentum communication tophat via a Ethernet, Modbus Plus, Devicenet, or other data network. In one example implementation, the ADU is tightly coupled to a programmable logic controller (PLC) such as a Momentum M1-E via a high speed proprietary serial interface, and the PLC can be programmed to continuously adapt the arc voltage threshold and hysteresis function in real time according to a real time adaptive algorithm.
The DSP illustrated includes 16 analog input channels that can be sampled and digitized by an integral 10-bit analog to digital converter 635. Signals presented to these analog input channels, such as the signals ICH 616 and VCH 614, to be discussed subsequently, can be sampled and digitized by the DSP at a user programmable rate. In one example implementation, this rate is programmable up to 10 kHz per channel. In another example implementation, a software program executed within the DSP provides for the selection and application of one of a plurality of digital finite impulse response filters to the sampled data signals. DSPC 630 also provides control signals to a Programmable Threshold Comparator function 620 to set the threshold and hysteresis values of the Programmable Threshold Comparator. In addition, DSPC 630 provides control and data paths to and from a high speed Arc Detector Logic Unit (ADLU) 640, which works in conjunction with Programmable Threshold Comparator 620 to accumulate arc statistics such as number of arcs and total arc time. DSPC 630 communicates with an external logic arrangement 60, such as a networked communication tophat or Programmable Logic Controller (PLC), via local data interface 70, for example a proprietary ATII interface. Examples of information that can be furnished from the ADU to the external logic arrangement 60 are the filtered chamber voltage and current, the number of individual arcing events and other values indicative of arc severity, as determined by Arc Detector Logic Unit 640. Examples of data that can be accepted by the ADU from the external logic arrangement are the instantaneous arc threshold voltage and hysteresis, and logical control signals that control the Arc Detector Logic Unit.
The fundamental sensed process inputs of Arc Detector Unit 50 are the differential output signals from the Vsense circuit (VPSIM+ and VPSIM−) and the Isense circuit (IPSIM+ and IPSIM−) of PSIM 40. Referring again to
The current filter portion of signal conditioner 610 that generates signal ICH from PSIM signals IPSIM+ and IPSIM− is identical in topology to that of the voltage filter, but the current signal equivalent to VCH′ is not used in the example embodiment. The output of the current filter, ICH is similarly band limited by an identical Butterworth filter with 3 dB crossover at approximately 2500 Hz.
Referring again to
VCS=0.6VCH+1.0 (Eqn. 1)
where VCS is that signal appearing on the non-inverting input, pin 4 of analog comparator U12:A in
Programmable threshold voltage signal, VTH, is provided to the inverting input of analog comparator U12:A (pin 5) to set the chamber voltage at which the ADU transitions between the NON_ARCING and ARCING states. A programmable hysteresis value, generated in a manner to be described subsequently, permits the value of VTH to be modal. A user specified value can be programmed to set the chamber voltage magnitude, VTHNA, at which the system transitions from the NON_ARCING to the ARCING state and a second voltage magnitude value, VTHAN, to set the voltage at which the system transitions from the ARCING to the NON_ARCING state. Device U13 is a dual 14-bit digital to analog converter (DAC), for example model AD5322 manufactured by Analog Devices, Inc., which is used to set the two values of VTH. It has two output terminals labeled, VOA and VOB, the voltage values of which are set by the DSP using a standard serial peripheral interface (SPI) feature, integral to the DSP. The signals labeled SPISIMO, SPICLK, .backslash.DAC1_SELECT and .backslash.LDAC are signals used by DSPC 630 to program a digital value ranging between 0 and 4095 for each of the two DAC channels. The precision 3.00 Volt reference described above is applied to U13, with the result that each DAC output generates an independent, analog output in the range 0-3.00 volt, in proportion to the ratio of the programmed digital value to the maximum value 4095. Output terminal VOB (pin 6), generated from the value of DAC B of U13 is coupled to the non-inverting input of operational amplifier U14:A, and is labeled VOB. As will be shown subsequently, the signal VOB determines the voltage threshold at which comparator U12:A transitions from the NON_ARCING to the ARCING state, VTHNA. The signal VOA, generated by the output of DAC A of U13 (pin 5) is coupled to the input terminal of analog switch U15:D, and as will also be shown subsequently, is used along with signal VOB to set the voltage threshold, VTHAN at which comparator U12:A transitions from the ARCING to NON_ARCING state. According to one example implementation, U15:D is part of a quad analog switch, for example DG201HS manufactured by Intersil and others. The output of this analog switch appears at pin 15 of U15:D and is labeled VSW in
The state transition threshold voltage VTH, is generated at output pin 1 of operational amplifier U14:A. Assuming an ideal operational amplifier U14:A, it is readily shown that the output signal VTH is related to signal VOB and the signal VSW by:
VTH=2VOB−VSW (Eqn. 2)
The instantaneous value of the signal VSW is dependent upon the logic state of the switch control input (pin 16) of U15:D. When the signal at switch control input (pin 16) of analog switch U15:D is in a logic “0” state, VSW follows signal VOA, generated by DAC U13 and connected to input terminal pin 14 of U15:D. When the control signal at switch control input (pin 16) of analog switch U15:D is in a logic “1” state, the circuitry driving output terminal, pin 15, of analog switch U15:D is placed in a very high impedance state and VSW closely follows VOB by virtue of the low resistance value of resistor R30 and the extremely small input bias current of operational amplifier U14.
The signal communicated to the switch control input of U15:D is provided by a logic OR gate U16:A. The input signals to OR gate U16:A are a hysteresis-enabling control output from DSPC 630 (.backslash.HYSEN) and the signal from the output of analog comparator U12:A, (pin 12). The logic state of signal .backslash.HYSEN is generated under DSP software control and is maintained in the logic “0” state under normal operation. The signal .backslash.HYSEN is set to a logic “1” state only during certain manufacturing system calibration and test procedures to isolate the hysteresis generating signal VOA from VSW.
As discussed earlier, the value of VSW and hence VTH is modal by virtue of the state of analog switch U15:D, which is dependent upon the state of digital signal .backslash.ARC at output terminal of analog comparator U12:A (pin 12). The relation between the signals VOA and VOB, both developed by DAC U13 and comparator threshold values VTHNA and VTHAN will now be derived. Assume first that the output signal of analog comparator U12:A is initially in a logic high state. This requires the level shifted chamber voltage signal, VCS on pin 4 of U12:A, to be at a higher level than the present threshold voltage, VTH on pin 5 of U12:A; by definition the NON_ARCING state. In said scenario, the output terminal of analog switch U15:D presents a high impedance and as discussed previously VSW is forced to take the value VOB by virtue of the low resistance value of R30 and the low input bias current of operational amplifier U14:A Under this condition, the signal at the output terminal of op amp U14:A follows VOB, and from Eqn. 2, VTH also takes the value VOB. Thus, voltage signal VOB directly sets the scaled, level shifted voltage at which comparator U12:A transitions from the NON_ARCING to the ARCING state, VTHNA according to:
VTHNA=VOB (Eqn. 3)
Once the scaled, shifted chamber voltage magnitude, VCS, drops below the programmed NON_ARCING to ARCING state transition value of threshold voltage VTH, VTHNA, generated according to Eqn 3, the signal at the output of comparator U12:A transitions from a logic “1” state (NON ARCING) to a logic “0” (ARCING) state. Assuming the .backslash.HYSEN control signal is in the logic “0” state (enabling the programmable hysteresis function), analog switch U15:D closes and the output of analog switch U15:D, VSW, follows the input of analog switch U15:D, VOA asserted by DAC A of U13, as discussed above. From Eqn. 2 with VOB set to VTHNA, the resulting threshold value VTH becomes:
VTH=2VTHNA−VOA (Eqn. 4)
If the programmed value of hysteresis (scaled to reflect the gains of the PSIM and level shifting network) is VHYSS, then setting VOA according to:
VOA=VTHNA−VHYSS (Eqn. 5)
and substituting into Eqn. 4 provides:
VTHAN=VTHNA+VHYSS (Eqn. 6)
Setting VOA according to Eqn. 5 allows the addition of a fixed hysteresis voltage value VHYSS to the NON_ARCING to ARCING state transition voltage VTHNA when the ADU is in the ARCING state to create the ARCING to NON_ARCING transition voltage value VTHAN. In summary, in this embodiment, DAC B output signal VOB is used to directly set the chamber voltage at which the programmable comparator transitions from the NON_ARCING to ARCING state according to Eqn. 1, while Eqn. 5 indicates an algorithm to determine a value for DAC A to add a hysteresis value to VTHNA to generate a related, but possibly higher transition voltage VTHAN from the ARCING to NON_ARCING state.
According to one implementation, the desired chamber threshold voltage value at which programmable comparator 620 transitions from the NON_ARCING to ARCING state and the desired voltage to be added to this chamber voltage threshold value to define the chamber voltage value at which the programmable comparator transitions from the ARCING to NON_ARCING state can be communicated to DSPC 630 from logic arrangement 60 via local data interface 70 and DSPC 630 can compute the correct digital values to send to DAC U13 to generate the appropriate signals VOA and VOB by virtue of affine transformations using appropriate scaling and offset constants stored integral to the DSP memory. In one example embodiment, to provide highly accurate threshold values, said scaling and offset constant values are computed for an individual module to account for the normal deviations from nominal values encountered in electronic components (e.g., resistor tolerance values) by virtue of a calibration routine. These calibration constant values are stored in a serial EEPROM integral to DSPC 630.
According to one example implementation, the sample rate of the analog to digital converters of DSP 630 is on the order of 10 kHz per channel, or one complete sample of the filtered chamber voltage and current signals, VCH and ICH every 100 uS. At this rate, a randomly occurring microarc of duration of 1 uS or less has a less than 1% probability of being detected by the DSP and, as discussed above, microarcs on the order of 1 uS are both common and can cause damage in integrated circuit manufacture. To reliably detect microarcs on the order of 1 uS or less in duration, ADU 50 includes high speed arc detector logic unit (ADLU) 640 that co-functions with the Programmable Threshold Comparator 620 and which can be controlled and monitored by DSPC 630 to generate statistical data regarding arcing during the PVD process. Referring to
According to one specific example, the ADLU includes interface means to DSPC 630 in the form of an address and data bus and accepts control signals from DSPC 630 such that DSPC 630 may read and write data from the device. The ADLU includes a register that permits DSPC 30 to control certain ADLU functions, such as resetting, enabling and disabling of counters, and also includes additional registers and control logic to permit DSPC 630 to read status information from the ADLU.
The signal presented at the D input terminal 1093 of the D flip-flop is driven by AND gate 1098. Input signals to AND 1098 are the signal ENB 1130 provided from the Counter Control Register 930, and the negation of signal .backslash.ARC 622 (.backslash . . . backslash.ARC 1150) from inverter 1097, signal .backslash.ARC 622 being provided by Programmable Comparator 620. When either the signal ENB 1130 is in the logic low (FALSE) state, or the .backslash.ARC signal is in the high state (indicating detection of a NON_ARCING chamber condition), the signal at the D input terminal 1093 is in the logic low state. Conversely, when the ENB signal is in the logic high state (thereby enabling counting), and the .backslash.ARC signal is in the logic low state (indicating detection of an ARCING chamber condition), the signal at the D input terminal 1093 is in the logic high state. Therefore, assuming counting is enabled (signal ENB 1130 is in a logic high state), the ACCLK signal 1160 will be in the logic low state on subsequent high-to-low transitions of the SYSCLK when the chamber is detected in a NON_ARCING condition. When an ARCING condition is detected, for example as indicated at 1180 in
ACC counter 1010 increments at each low-to-high transition of the signal at its CLK input terminal whenever the CRST signal is asserted low. Thereby, ACC counter 1010 effectively counts the quantity of chamber transitions from the NON_ARCING condition to the ARCING condition, while the ENB signal is asserted high (enabling the counting). In the example embodiment, ACC counter 1010 can resolve microarcs detected by the Programmable Comparator 620 (generating the .backslash.ARC signal) as short as 33 nS using a SYSCLK signal having a frequency on the order of 30 MHz. Higher resolution can be achieved by increasing the clock rate.
ATC counter 1020 is used to estimate the total time the chamber is in the ARCING condition as determined by Programmable Comparator 620. ATC counter 1020 increments at each low-to-high transition of the signal at its CLK input terminal whenever the CRST signal is asserted low. The CLK input terminal of ATC counter 1020 is driven by signal ATCLK 1170 provided by AND gate 1094 having ACCLK and SYSCLK signal inputs. Signal ATCLK 1170 begins tracking the SYSCLK signal 1110 whenever counting is enabled (ENB signal 1130 is high) and a chamber ARCING condition is detected (.backslash.ARC signal 1140 is low), for example at 1186 in
The ACC 1030, ATC High 1040 and ATC Low 1050 latching snapshot registers permit the ACC counter 1010 value, the ATC counter 1020 high order word, and the ATC counter 1020 low order word values to be captured respectively, on command at an instant in time. This permits DSPC 630 to read the state of the counters at a specified instant, holding those values for subsequent retrieval by DSPC 630, while permitting the ACC and ATC counter to continue to operate according to their respective logic described above. Each of these three 16-bit registers is arranged and configured to capture the instantaneous corresponding counter value on a low-to-high transition of the SNP signal, provided by Counter Control Register 930 under control of DSPC 630 as will be discussed. The output signal of each of the snapshot registers are 3-state buffered to an internal data bus 950 by the ACC 1060, ATC High 1070 and ATC Low 1080 3-state buffers respectively. The DSP Interface Logic 960 asserts an enable signal on RACC 1086 to ACC 3-state buffer 1060 in order to provide the captured value of ACC latching snapshot register 1030 on internal bus 950; asserts an enable signal on RATH 1087 to ATC High 3-state buffer 1070 in order to provide the captured value of ATC High latching snapshot register 1040 on internal bus 950; and asserts an enable signal on RATL 1088 to ATC Low 3-state buffer 1080 in order to provide the captured value of ATC Low latching snapshot register 1050 on internal bus 950.
Referring again to
Referring again to
Address decoding to generate the control signals for ADLU 640 is functionally provided by an address decoder, for example, a 2-to-4 binary address decoder 1260 as shown in
With the decoder logic as set forth above, Table 2 defines the logic generating the signals at each of the function select outputs in
The processing of signals ICH and VCH generated by Analog Signal Conditioner 610 is now discussed in greater detail. Referring again to
NICH(n)=FIX((ICH(nT)/VREF)*1024) (Eqn 8)
and
NVCH(n)=FIX((VCH(nT)/VREF)*1024) (Eqn 9)
Where the function FIX(arg) truncates the value of its argument “arg” to the nearest integer, n denotes the nth sample taken by DSPC 630 from a reference time and T is the sample period. In one particular embodiment, the DSP is programmed to convert the analog signals VCH and ICH at a rate of 10 kHz, resulting in sampled data sequences of numbers {NVCH} and {NICH} responsive to the chamber voltage and current. In one particular embodiment, software internal to the DSP provides for the application of user selectable digital finite impulse response (FIR) filters to the sequences, resulting in filtered sequences {FVCH} and {FICH} respectively, although other signal processing techniques can be applied to the sequences without loss of generality. In one particular embodiment, an affine transformation is applied to the sequences {FVCH} and {FICH} resulting in sequences of numbers {SFVCH} and {SFICH} that are a scaled integer estimate sequences of the chamber voltage and current. In one example, the affine transformations are such that a continuously applied chamber voltage of 1000 volts results in the generation of a sequence of integers each with value 1000, with other voltage values scaled proportional. Similarly, in this example, the affine transformation applied to the sequence derived by sampling and converting the ICH signal takes into consideration the various gains and offsets of the PSIM and Analog Signal Conditioning circuits, resulting in a transformation in which a current of 10.00 Amperes appears as the integer 1000, with other values proportional.
In one example implementation, the present value of the sequences are communicated via high speed communication interface 70 to logic arrangement 60 where logic arrangement 60 uses the present and past values to compute an adaptive arc threshold voltage value to be used by Programmable Threshold Comparator 620. This adaptive arc threshold voltage value and desired hysteresis level is subsequently communicated from logic arrangement 60 back to DSPC 630 via High Speed Communication Interface 70. DSPC 630 then converts the desired threshold values to the appropriate DAC values according to the operation of Programmable Threshold Comparator 620. This approach results in a near real time adaptive threshold. In another example implementation, the algorithms to generate the adaptive threshold reside in DSPC 630 itself, resulting in an adaptive voltage threshold with minimal delay.
One example algorithm to generate an adaptive arc voltage threshold is to base the computed threshold on a moving average of the voltage sequence computed by DSPC 630, the length of the moving average chosen to be long compared to the expected duration of an arc, but short with respect to the period of rotation of the steering magnet. At a 10 kHz sample rate, the moving average can be computed using a uniformly weighted 64 point FIR filter, the sequence at the filter output representing the average of the previous 6.4 mS of voltage measurements. In one implementation, the adaptive arc threshold value is computed by subtracting a fixed voltage from the moving average. In another example implementation, the adaptive threshold is computed as a fixed percentage of the moving average.
These filtered, transformed sequences can also be used to provide further information indicative of the overall health of the process. In one example, multiplying the instantaneous value of the current sequence with the instantaneous value of the voltage sequence provides an instantaneous power sequence that can be used to verify that the actual power delivered to the vacuum chamber is that delivered by the power supply. Such a sequence can be used to determine, for example, that a cable breakdown is occurring, shunting current around the vacuum chamber. Another example of the use of these sequences is that they can be used as an independent means to estimate the rotational speed of the steering magnet. As described above, it has been observed that the chamber voltage and current vary periodically with the steering magnet period as the chamber impedance varies due to geometric and other considerations. In one example, the scaled voltage or current sequence is passed through a digital high pass filter to remove the DC component. The resulting AC sequence is then tracked by a digital phase locked loop, from which the rotational frequency of the steering magnet is estimated. In another example implementation, a discrete Fourier transform is applied to the voltage or current sequence, and the magnet rotation frequency determined from the resulting spectrum. If the estimated rotation speed differs significantly from the expected rotation speed, a mechanical or electrical problem may be the cause. This information can be used to detect an incipient fault in the mechanical or electrical system.
According to another example embodiment of the present invention, the components and operation described above are replicated for monitoring multiple chambers or for detecting ARCING based upon additional threshold values applied to a single chamber voltage and current signals. In a particular example embodiment, four independently operating ADU functions controlled by a single DSPC 630 are provided. The four chamber version of the ADU can be configured to simultaneously monitor four independent chambers via four PSIMs, or a single PSIM can drive multiple ADU chamber inputs by wiring the corresponding VPSIM+, VPSIM−, IPSIM+ and IPSIM− ADU input signals for multiple chambers in parallel. In an example embodiment, when all four ADU functions are monitoring a single chamber via a single PSIM and wired in this manner, four different threshold values can be programmed for a single chamber. A count of number of arcs and arc duration at each programmed threshold value is maintained by the combination of corresponding Programmable Comparator 620 and ADLU 640. In the embodiment, DSPC 630 has access to all four ADLU functions, and arcing conditions can be resolved into one of four levels corresponding to the four independently programmed thresholds.
For instance, in a system employing four independent monitors attached to a single PSIM per above, and voltage threshold magnitudes programmed at 100, 200, 300 and 400 volts, a single arc having a minimum voltage magnitude of 250 volts will appear on the monitors with thresholds programmed at 300 and 400 volts, but not on those programmed at 100 and 200 volts. Furthermore, if the system is capturing a single arc in this manner, the period over which the chamber voltage collapses below the 300 volt level will appear simultaneously in the ADLU arc time counters corresponding to the 300 and 400 volt level, while the period over which the chamber voltage collapse is between 300 and 400 volts will appear only on the ADLU arc time counter corresponding to the 400 volt level. The arc event can then be resolved into two arc times—the arc time spent between 200 and 300 volts, read directly from the arc time counter of the ADLU corresponding to the 300 volt threshold, plus the arc time spent between 300 and 400 volts, computed by taking the difference between the ADLU arc time counters corresponding to the 400 volts and 300 volts respectively. This algorithm can be repeated as required for other arcs of different intensities.
In one particular example implementation, DSPC 630 samples the four ADLU register sets at a 10 kHz rate and communicates the arc count and arc time count for all four channels via High Speed Communication Interface 70 to Logic Arrangement 60. DSPC 630 also samples and transfers the nominal, filtered chamber current ICH, and filtered chamber voltage VCH to Logic Arrangement 60, which performs the mathematical operations required to resolve the arc per above and compute an estimate of arc energy. All four arc voltage threshold values can be computed adaptively by extension of the discussion above. In another example embodiment, DSPC 630 performs the computations internally, transmitting the resulting estimate of arc related parameters, such as arc time at each threshold value, and estimated arc energy to Logic Arrangement 60.
According to one example implementation, the logic arrangement 60 is an external logic arrangement, for example a programmable logic controller (PLC), tophat, or similar computing device. According to a more particular embodiment, the logic arrangement 60 is a Schneider Automation M1-E PLC. According to one aspect of the present invention, the ADU is incorporated into a Momentum form factor and adapted to communicate with Momentum tophats and programmable logic controllers (PLCs).
In one implementation, the data collected by the logic arrangement 60 is recorded. Software running on the logic arrangement 60 logs data, graphs data, and can provide network-based alarms responsive to the data. A system controller provides real-time control of the plasma generation application. When the arc count and/or arcing duration exceeds a selected quantity per deposition, the logic arrangement 60 determines according to a pre-defined algorithm that the arcing is damaging the substrate during material deposition, and communicates with the system controller to terminate the deposition. The logic arrangement 60 can also indicate that the substrate being processed will have reduced yield due to the arcing.
In addition to counting arcs and the cumulative duration of arcing for each deposition, the logic arrangement 60 is used to perform other real-time analysis of arc information in other implementations. For instance, analysis such as recording the total number (and duration) of arcs for the target, recording the arc intensity (referring to the proximity to ground potential, indicative of a direct short), and detecting continual arcing, which indicates a potential defect in the target requiring complete tool shut down for repair. In another implementation, a system controller provides a signal based on arcing rates, on arcing durations, on rate of change of arc rate/durations, or based on arc “quality,” arc quality being proportional to duration, quantity and arc intensity (i.e., magnitude) or severity (as measure for example, by a product of the arc duration and magnitude).
According to another example embodiment of the present invention, a method integrates an arc detector with hardware necessary to inform a user, in real-time, that there is a problem with the sputtering source and that the newly-processed wafer may have reduced yield. Accordingly, various embodiments of the present invention can be realized to provide arc detection in other plasma generation control applications, such as for case hardening steel, among others. Generally, the circuit arrangements and methods of the present invention are applicable wherever a plasma generation chamber or its equivalent might be implemented.
According to another embodiment of the invention, the arc detector unit 50′ (as shown in
One major application of this process is the deposition of a metal layer on a silicon wafer substrate in the manufacturing of integrated circuits (ICs). As discussed above, this process is prone to “arcing.” Arcing ejects macro-particle contaminants from the target. Some of this contaminant material can land on the wafer, causing product defects and nonuniformities in the coating that negatively affect manufacturer's revenues. Arcing may be caused by (i) target impurities or inclusions, (ii) target or kit aging and physical tolerance changes, or (iii) wafer alignment.
Arcing during the PVD process results from an unintended low impedance path from the anode to the target. When an arc occurs, the magnitude of the chamber impedance decreases rapidly, usually too fast for the power supply to respond. A rapid drop in the magnitude of voltage between the anode and cathode of the chamber can be observed. As a result, comparing the chamber voltage to a threshold value can provide early detection of arcs. Through such early detection, manufacturers can address root causes of arc generation without incurring excessive loss of revenue due to arc-generated defects.
As illustrated in
The PSIM 40 is used to convert high-voltage and high-current readings from the power supply 30 into a 0-10V range for input to the ADU 50′. The 0-10V signals are linearly proportional to the chamber voltage and current. This provides a voltage signal indicative of the power supply 30 voltage, and a current signal indicative of the power supply 30 current.
Referring to
The programmable logic controller (PLC) or other logic arrangement or circuitry 60, among its many functions, reads data from the ADU, converts DSP clock cycles into microseconds, resets arc counters, and makes the data available on ethernet. The PLC also sends command parameters to the ADU—when to look for arc events, what the threshold value is, whether the excursion should be above or below threshold, etc.
The ADU 50′ has four transient-monitoring (arc) channels, and four auxiliary channels. (The auxiliary channels may be used to record associated data but are not capable of counting arcs. The auxiliary channels are available for data collection in an upgraded system.). Referring to
For the arc channels 2700/2702 and 2704/2706, signal propagation and filtering is as shown in
Preferably, the factory-standard PLC 60 used in this embodiment is a Momentum™ MIE 96030 processor. Signal connections to the ADU 50′ (from the PSIM BNC connectors) are made with RG-178 coaxial cable via a standard 18 pin Momentum™ connector. The PLC 60 interfaces to the ADU 50′ via a standard Momentum™ ATII hardware interface. The ATII interface supports 32 registers in each direction. The b32 registers are segmented into four identical groups of 8 registers, one for each channel.
The PLC 60 operates by the principle of scan cycles. During one scan cycle, the PLC 60 executes each of its instructions once and refreshes its I/O registers (through which it communicates with the ADU 50′) once. Therefore, the PLC 60 controls the ADU 50′ by first reading the 32 Status Registers and writing the 32 Command Registers, then running its own control program based on the newly-read data from the Status Registers. The PLC program contains logic that repeats four times, once per ADU channel (master voltage, master current, slave voltage, and slave current). The program also contains logic that is performed on each pair of channels (master and slave) because it incorporates data from both channels (current and voltage) per power supply.
When the PLC 60 reads the Status Registers (8 registers per ADU channel) from the ADU 50′, the PLC program relies on four primary pieces of data per ADU channel. The four variables are the Status Register (in particular, bit 9, whether or not the ADU is at that moment measuring an arc), PSIM Signal, Arc Counts, and Arc Time. Note that the PSIM Signal 1714 (as shown in
Referring to the block diagram of
The ARC COUNTS & TIME CLASSIFICATION logic section 1808 takes the latest Arc Counts and Arc Time readings 1810 from the ADU 50′ and if new Arc Counts and Arc Time have appeared, adds them to one of the three PLC Arc Counts and Time categories (only if status bit 9 of the Status Register must read logic false, indicating that the ADU 50′ was not between beginning and ending of an arc). The three categories are Stable, Transition Rising, and Transition Falling. If the Stable Flag is logic true, new Arc Counts are added to the Stable Arc Counts total and new Arc Time is added to the Stable Arc Time total. If the Stable Flag is logic false, the PLC 60 tracks whether the Transition is Rising or Falling. Depending on which Transition is occurring, Arc Counts and Arc Time are added to the appropriate Arc Counts Transition and Arc Time Transition totals, either Rising or Falling.
It is important to note that the reason for Stable and Transition modes is that the present system does not know beforehand when a power supply step change or shut-down will occur. Because the ADU 50′ is looking for arcs on the voltage channels as points where voltage drops below a threshold, it will always generate an Arc Count and some Arc Time until the next PLC scan cycle when it can reduce the threshold or disable the ADU 50′ from counting arcs. Arc Time from power-down events is typically much greater than Arc Time during true microsecond arc events. Therefore, without a priori recipe or step duration information, the PLC 60 goes into Transition mode and separates Arc Time found during Transitions from Arc Time found during Stable processing. The Transition Hold Delay 1902 is intended to provide a blanking period during which plasma ignition transients may settle down. The Transition Hold Delay 1902 parameter and the Transition modes are methods of reducing false positives in the data.
A Unit Conversion section 1812 simply multiplies the PSIM Signal by the corresponding Calibration Constant and Calibration Percent values to convert the 0-10000 mV PSIM Signal into real-world engineering units of Volts or Amps (e.g., 0-10V as discussed above). The Calibration Constant parameter is a function of the PSIM hardware and should only be changed if the PSIM voltage divider resistors change or if the current transducer and its gain change. The Calibration Percent parameter is intended to be adjusted if it is desired to match voltage and current (or power) readings with similar readings from a different source, from the PVD equipment itself or other components.
The EWMA filter 1710 provides a method to track the PSIM Signal. The output 1814 of the EWMA filter is used to adjust Threshold, SUB, and SLB. In Stable mode, the tracking is slow so that the four adjusted parameters may slowly adapt to drifts or slow changes in the PSIM Signal setpoint. In Transition mode, the tracking is faster because the setpoint has changed and the PSIM Signal is quickly ramping up or down in order to achieve the new setpoint level. The equation governing the EWMA filter is given by
y(k)=λ/100*PSIM Signal(k)+(1−λ/100)*y(k−1) (1)
where y is the is the value at the output of the filter, k is the PLC scan cycle index (each time the PLC begins a new scan, k increments by 1), and λ is the filter coefficient. If the Stable Flag is logic true, λ is the Stable Filter Coefficient. If the Stable Flag is logic false, λ equals the Transition Filter Coefficient. Note that λ may take a value between 0 and 100, representing a percentage level. The closer λ is to 100, the more the most recent reading, PSIM Signal(k), affects the filter output, y(k), and the filter more quickly tracks a rapidly changing voltage or current level. The closer λ is to 0, the less the most recent sample of PSIM Signal affects y(k), the more the exponentially decaying average of previous samples of PSIM Signal determine y(k), and the filter will very slowly follow a step change in the PSIM Signal. Note that PSIM Signal is interpreted such that both voltage and current are positive quantities (i.e. chamber voltage, while measured from cathode to anode, is an absolute value quantity rather than a negative value).
A YTH AND YHYS COMPUTATION section 1816 takes the output of the filter 1710, y, and calculates the threshold value to write to the ADU 50′ in the next read/write phase of the PLC scan cycle. The threshold level is equal to y multiplied by the appropriate percentage as determined by the Stable Flag, either Stable Threshold Percentage or Transition Threshold Percentage.
In a STABLE BAND COMPUTATION 1818, the output of the EWMA filter 1710, y, is multiplied by the Stable Band Percentage and then added to y to result in SUB and subtracted from y to yield SLB. If the product of y and Stable Band Percentage is less than Stable Band Minimum (SBM), then SBM is added to y and subtracted from y to generate SUB and SLB, respectively.
A ENB/CRST BLOCK 1820 performs three functions: (1) tells the ADU 50′ whether or not to look for arcs, (2) resets the ADU 50′ at the end of the wafer 1310, and (3) keeps track of overall process time. To perform the first function, the ENB/CRST BLOCK 1820 sets the ADU Control Register Enable bit, ENB (Bit 1) to high when PSIM Signal is greater than the Enable Level. The ENB bit is set to low as soon as PSIM Signal is less than Enable Level. The second function is to reset the ADU 50′, which is done when the ADU 50′ is not enabled for a time that reaches the Reset Delay. In most PVD processes, the time between wafers 1310 exceeds the time of recipe steps during wafer processing where the power supplies are off. Therefore, to appropriately reset the ADU 50′ between wafers 1310, the Reset Delay should be set to a value (in seconds) greater than intra-recipe power-off times, and less than between-wafer power-off time. The third function is that of tracking total process time, from first power-on to last power-off. After the aforementioned sections of logic are completed (and the other logic sections as well, though they do not affect any of the variables written to the ADU 50′), Threshold and Control Register are written to the ADU 50′ in the next read/write phase of the scan cycle.
Additional logic performed by the PLC 60 is shown in
Arc Counts and Time from the Voltage channel 2202 and Current channel 2204 are fed into the ARC CLASSIFICATION logic section 2206. If Arc Counts on both Voltage and Current channels 2202, 2204 show an increase since the last PLC scan, regardless of their corresponding Arc Times (and the ARC bit, Status Register, Bit 9 is not high), the PLC 60 increments the Arc Class 1 counter and calculates Scan Energy given by
Scan Energy(k)=[yv(k)−YVTH(k)]*[YITH(k)−yI(k)]*[tarcV(k)+tarcI(k)]/2 (2)
where k is the PLC scan cycle index, yV is the EWMA filter output for the voltage channel, YVTH is the Threshold value for the voltage channel, yI is the EWMA filter output of the current channel, YITH is the Threshold value for the current channel, tarcV is the Arc Time for the voltage channel (for the latest PLC scan rather than cumulative), and tarcI is the Arc Time for the current channel (again for the latest PLC scan). Scan Energy is essentially the product of the area under the voltage curve and the area under the current curve where they deviate from their nominal (or EWMA-filtered) values. The time factor in the Scan Energy calculation is the average of the time seen on the two channels. Arc Energy is the cumulative sum of Scan Energies. If only Voltage Arc Counts have changed since the last scan, the Arc Time is checked relative to a boundary at 500 μs. (This boundary is hard-coded in the PLC). If Arc Time is less than the boundary value, the Arc Class 2 counter is incremented. If Arc Time is greater than or equal to the boundary value, the Arc Class 3 counter is incremented. If only Current Arc Counts register since the last PLC scan, Arc Time is checked and the Arc Class 4 or Arc Class 5 counter is incremented, depending on whether the Arc Time is less than the boundary or greater than or equal to the boundary value, respectively. The physical interpretation of each of the five classes is given in
To summarize how the PLC program functions, a timing diagram is given in
Lastly, the adaptation of Threshold relative to process Voltage is shown in
In stand-alone mode, where the system has no information as to when process transitions occur, it is normal to see Falling Transition Arc Counts and Time on the Voltage channels. It is also normal to see Rising Transition Arc Counts and Time on the Current channels. In both cases, the signals suddenly move in the direction in which the ADU 50′ is looking for sudden transients. Until the PLC 60 (at 30 Hz) can catch up with the ADU 50′ (30 MHz) and give the command to change threshold, the ADU 50′ will count step changes as arcs. Hence the need to separate data into Stable and Transition components.
System parameters should be adjusted such that Arc Counts, Arc Time, Arc Energy, Arc Classes, Process Transitions, Ignition Time and Process Time data (and the rest of the Output Data, but the aforementioned set comprises the critical data points) are all being optimally reported by the system. The goal is to capture data for “true arcs” that affect wafer quality.
The various variables and parameters involved in the system are graphically illustrated in
In the Power and Ignition Time path, voltage and current for the master power supply PSIM (or the slave) are combined to yield Calculated Power, which is in turn used to calculate Ignition Time. The parameters used in the calculation of each variable are shown in italics in
The ADU Enable path determines when the ADU is actively looking for arcs. It is controlled by the Enable Level and Enable Delay parameters. When PSIM Signal rises above Enable Level, the PLC will command the ADU 50′ to begin looking for arcs by setting the Enable Bit in the Control Register. Enable Level should be above the off-state reading and below the lowest Voltage or Current level (in equivalent PSIM Signal units) seen in the recipes run by the PVD tool. Enable Delay may be increased if it is desired to keep the ADU 50′ inactive for a time period after PSIM Signal rises above Enable Level. Practically speaking, the Enable Level condition (combined with the Stable vs. Transition modes) is sufficient in and of itself, therefore Enable Delay will likely never need to be adjusted in PVD applications.
In the Threshold path, PSIM Signal is fed into an EWMA filter governed by Stable Filter Coefficient or Transition Filter Coefficient, as determined by the state of Stable Flag. The Filter Coefficient parameters may vary from 0 to 100. High values increase the bandwidth of the filter which permits quick-response tracking of step changes (yet poor noise rejection). When in steady-state, or during Stable Mode, the system should be set to have a Threshold (remember Threshold=Threshold Percentage*Filter Output) where noise in the Threshold coupled with noise in the DC Power Supply signal will not lead to false arc counts.
The output of the Filter is then fed into the Stable Upper Band/Stable Lower Band (SUB/SLB) calculation where the Filter Output is multiplied by the Stable Band Percentage. If this product is less than the Stable Band Minimum parameter, then Stable Band Minimum is added to and subtracted from Filter Output to yield SUB and SLB, respectively. Otherwise, the product is added to and subtracted from Filter Output to give SUB and SLB. SUB and SLB are then used in the beginning of the next PLC scan to determine whether the Stable Flag is true or false. Stable Band Percentage should be low enough to ensure that the smallest step changes between recipe steps cause the system to enter Transition Mode, yet it should be high enough so that any power loss events that may be present do not cause the system to enter Transition Mode in the middle of what should be a single recipe step. Stable Band Percentage may initially be set from known recipe voltage and current profiles, but must be verified empirically by examining data from multiple wafer runs for each process. Stable Band Minimum should be set such that when the system is in power-off state, Stable Flag does not change back and forth between true and false. It may be set simply by observing off-state noise and tripling observed variation.
Filter Output is also fed into the Threshold calculation. The Stable Flag determines which mode applies, Stable or Transition. Threshold is then Filter Output multiplied by Stable Threshold Percentage or Transition Threshold Percentage. Threshold is one of the two most important variables in the data.
Threshold, through Stable Threshold Percentage, should be adjusted up and down during steady-state operation of the various process recipes and power setpoints to identify power supply ripple, keeping in mind that ripple will vary with time and from chamber to chamber. Stable Threshold Percentage must be set such that Threshold is well below power supply ripple, yet sufficiently high to capture short-duration arcs. Recall that the PSIM contains a 60 kHz filter in its circuitry (by design to mitigate the effects of power supply switching noise), for which the time constant is 2.6 μs. Given that voltage transients for true arcs take voltage from process setpoint to zero in much less than 1 μs, and assuming an arc may be represented by a square-wave function (two step changes of opposite direction and equal magnitude), the time constant and Stable Threshold Percentage will determine the shortest arc detectable by the system. For example, in
Therefore, Stable Threshold Percentage should be set well below the level where the ADU counts noise as arc events, yet not so low that a large percentage of true arc events do not cause PSIM Signal to cross Threshold. Transition Threshold Percentage should be set similarly, keeping in mind that ignition periods are inherently noisy, therefore its value will likely be less than that of Stable Threshold Percentage. Transition Hold Delay may be used to lengthen or shorten the period during which Transition Threshold Percentage applies.
In the Process Time path, the only calculation is that of Process Time itself. Process Time is the time from when PSIM Signal exceeds Enable Level to the point where PSIM Signal falls below Enable Level and remains below for at least a time equal to Reset Delay (Note that Reset Delay is subtracted from Process Time when it is exceeded so that Process Time reflects the difference between first ADU Enable true condition and last ADU Enable true condition for the wafer.). The system's reset logic may be replaced in the PLC program by a signal another device indicating that the wafer process has ended and that data may be reset, thereby rendering Reset Delay unnecessary.
Finally, the Stable Flag/Arc Counts/Arc Energy/Arc Class path contains the last two parameters, Transition Hold Delay and Arc Class Boundary. Stable Flag again is one of the two most important system variables. It is true if PSIM Signal falls within the range defined by SUB and SLB from the previous PLC scan. Otherwise, it is false and remains false until PSIM Signal again falls with the SUB-SLB range for the Transition Hold Delay period of time. Stable Flag affects the EWMA Filter, Threshold, and the binning of Arc Counts into Stable, Rising Transition and Falling Transition categories. To adjust Transition Hold Delay, adjust its value and compare all three categories of Arc Counts data during and immediately after Stable Flag is false. If Stable Arc Counts are occurring regularly immediately after Stable Flag becomes true at the beginning of the process or of a step, Transition Hold Delay should be increased.
While the specific embodiments have been illustrated and described, numerous modifications come to mind without significantly departing from the spirit of the invention, and the scope of protection is only limited by the scope of the accompanying Claims.
Claims
1. A method of detecting and classifying arcs in a physical vapor deposition process comprising:
- monitoring a power supply voltage and current of a plasma generation apparatus;
- detecting a plurality of instances when the voltage drops below a predetermined first voltage threshold;
- timing the duration of each instance the voltage drops below the predetermined first voltage threshold;
- detecting a plurality of instances the current spikes above a predetermined first current threshold;
- timing the duration of each instance the current spikes above the predetermined first current threshold; and
- identifying each instance the voltage drops below the predetermined first voltage threshold with and without a corresponding coincidental current spike and each instance the current spikes above the predetermined first current threshold with and without a corresponding coincidental voltage drop; and
- sending data for each of the identified instances.
2. The method of claim 1 further comprising the steps of:
- determining whether the voltage is in one of a stable mode, a rising transition mode and a falling transition mode.
3. The method of claim 2 further comprising the steps of:
- maintaining a count of arc events and corresponding durations occurring when the voltage is in a stable mode;
- maintaining a count of arc events and corresponding durations occurring when the voltage is in a rising transition mode; and,
- maintaining a count of arc events and corresponding durations occurring when the voltage is in a falling transition mode.
4. The method of claim 1 further comprising the step of:
- classifying arc events based on monitoring the power supply voltage and current of the plasma generation apparatus.
5. The method of claim 1, further comprising the steps of:
- for each of the identified instances where a voltage drop and a current spike are coincidental, assigning that identified instance a first classification;
- for each of the identified instances where one or more voltage drops without a corresponding coincidental current spike have a cumulative duration less than a predetermined time, assigning that identified instance a second classification;
- for each of the identified instances where one or more voltage drops without a corresponding coincidental current spike have a cumulative duration greater than a predetermined time, assigning that identified instance a third classification;
- for each of the identified instances where one or more current spikes without a corresponding coincidental voltage drop have a cumulative duration less than a predetermined time, assigning that identified instance a fourth classification; and,
- for each of the identified instances where one or more current spikes without a corresponding coincidental voltage drop have a cumulative duration greater than a predetermined time, assigning that identified instance a fifth classification,
- wherein sending the data comprises sending data that indicates the assigned classification of each of the identified instances.
6. The method of claim 1 further comprising the step of:
- calculating the scan energy for each of the identified instances having a first classification.
7. The method of claim 1 further comprising the steps of:
- disabling detecting a voltage drop below a predetermined first threshold for a transition hold period after each detection of a voltage drop below the predetermined first threshold; and,
- disabling detecting a current spike above a predetermined first threshold for a transition hold period after each detection of a current spike above the predetermined first threshold.
8. The method of claim 1 further comprising the step of:
- adjusting the predetermined first voltage threshold during a scanning cycle to track slow changes in the supply voltage.
9. The method of claim 1 wherein the duration of voltage drops and the duration of current spikes are measured in clock cycles.
10. The method of claim 1 further comprising the steps of:
- detecting each arc event instance where the voltage drops below a predetermined second voltage threshold;
- detecting each arc event instance the current spikes above a predetermined second current threshold.
11. A method of determining an arc event in a plasma generation apparatus comprising the steps of:
- monitoring a power supply current and voltage;
- detecting a voltage drop in the monitored voltage;
- determining whether a current spike in the monitored current coincides with the voltage drop;
- determining that a first arc event type has occurred if it is determined that a current spike in the monitored current coincides with the voltage drop;
- determining that a second arc event type has occurred if it is determined that a current spike in the monitored current does not coincide with the voltage drop, and
- sending data based on the event type.
12. The method of claim 11 further comprising the step of:
- calculating a scan energy associated with the voltage drop.
13. The method of claim 11 further comprising the step of:
- calculating an arc energy wherein the arc energy is a cumulative sum of a plurality of scan energies associated with a plurality of voltage drops in the monitored voltage.
14. A method for detecting arcs in a plasma generation apparatus having an interface configured to detect a voltage and current of a supply of power, the plasma generation apparatus being configured to create an ionized gas between a target and a wafer, the method comprising the steps of:
- comparing the voltage to a voltage threshold value at a set frequency; and,
- comparing the current to a current threshold value at the set frequency;
- first determining if an arc event occurred based on a first comparison of the voltage to the voltage threshold value and a first comparison of the current to the current threshold value; and
- second determining if another arc event occurred based on a second comparison of the voltage to the voltage threshold value and a second comparison of the current to the current threshold value, wherein responsive to the first determining that an arc event occurred, performing the second determining comprises delaying the second comparison of the voltage to the voltage threshold value and the second comparison of the current to the current threshold value for a transition delay period.
15. The method of claim 14, further comprising the steps of:
- generating a power-related parameter;
- comparing the power related parameter to at least one threshold to determine the severity of arcing in the plasma generation apparatus; and,
- measuring arc duration responsive to comparing the power related parameter to the at least one threshold.
16. The method of claim 14, further comprising sending data if an arc event is determined to have occurred.
17. An apparatus for detecting an arc event in a plasma generation chamber, the apparatus comprising:
- a power supply interface module configured to detect a power supply voltage and current applied to the plasma generation chamber;
- an arc detection unit communicatively coupled to the power supply interface module, the arc detection unit including a threshold comparator circuit arranged to compare the voltage to a first voltage threshold value and compare the current to a first current threshold value; and
- logic circuitry configured to make a determination of events based on an output of the threshold comparator circuit, to determine the duration of events based on the voltage dropping below the first voltage threshold value and the duration of an event based on the current spiking above the first current threshold value, and to classify events based on the output of the threshold comparator circuit and the duration of each event by: for each of the events where a voltage drop and a current spike are coincidental, assigning that event a first classification, for each of the events where one or more voltage drops without a corresponding coincidental current spike have a cumulative duration less than a predetermined time, assigning that event a second classification, for each of the events where one or more voltage drops without a corresponding coincidental current spike have a cumulative duration greater than a predetermined time, assigning that event a third classification, for each of the events where one or more current spikes without a corresponding coincidental voltage drop have a cumulative duration less than a predetermined time, assigning that event a fourth classification, and for each of the events where one or more current spikes without a corresponding coincidental voltage drop have a cumulative duration greater than a predetermined time, assigning that event a fifth classification.
18. The apparatus of claim 17 wherein the threshold comparator circuit is programmable to enable a user to set an initial voltage threshold value and an initial current threshold value.
19. The apparatus of claim 17 wherein the logic circuitry is arranged to determined if the voltage is in one of a stable mode, a rising transition mode and a falling transition mode.
20. The apparatus of claim 19 wherein the logic circuitry is arranged to maintain a count of the events occurring when the voltage is in a stable mode, maintain a count of the events when the voltage is in a rising transition mode, and maintain a count of the events when the voltage is in a falling transition mode.
21. The apparatus of claim 17, wherein the logic circuitry is arranged to compute arc energy.
22. The apparatus of claim 17, wherein the arc detection unit includes a digital signal processor.
23. A method of determining an arc event in a plasma generation apparatus, the method comprising the steps of:
- monitoring a power supply current and voltage;
- detecting a current spike in the monitored current;
- determining whether a voltage drop in the monitored voltage coincides with the current spike;
- determining that a first arc event type has occurred if it is determined that a voltage drop in the monitored current coincides with the current spike;
- determining that a second arc event type has occurred if it is determined that a voltage drop in the monitored current does not coincide with the current spike, and
- sending data based on the event type.
24. The method of claim 23, further comprising calculating a scan energy associated with the current spike.
25. The method of claim 24 further comprising calculating an arc energy, wherein the arc energy is a cumulative sum of a plurality of scan energies associated with a plurality of current spikes in the monitored voltage.
26. A plasma generation apparatus, comprising:
- a power supply interface configured to monitor a power supply voltage and current of the plasma generation apparatus;
- an arc detector configured to detect a plurality of instances when the voltage drops below a predetermined first voltage threshold and a plurality of instances when the current spikes above a predetermined first current threshold; and
- logic circuitry configured to: time the duration of each instance the voltage drops below the predetermined first voltage threshold, time the duration of each instance the current spikes above the predetermined first current threshold, and identify each instance the voltage drops below the predetermined first voltage threshold with and without a corresponding coincidental current spike and each instance the current spikes above the predetermined first current threshold with and without a corresponding coincidental voltage drop.
27. The plasma generation apparatus of claim 26, wherein the logic circuitry is further configured to determine whether the voltage is in one of a stable mode, a rising transition mode and a falling transition mode.
28. The plasma generation apparatus of claim 27, wherein the logic circuitry is further configured to:
- maintain a count of arc events and corresponding durations occurring when the voltage is in a stable mode;
- maintain a count of arc events and corresponding durations occurring when the voltage is in a rising transition mode; and,
- maintain a count of arc events and corresponding durations occurring when the voltage is in a falling transition mode.
29. The plasma generation apparatus of claim 26, wherein the logic circuitry is further configured to classify arc events based on monitoring the power supply voltage and current of the plasma generation apparatus.
30. The plasma generation apparatus of claim 26, wherein the logic circuitry is further configured to:
- for each of the identified instances where a voltage drop and a current spike are coincidental, assign that identified instance a first classification;
- for each of the identified instances where one or more voltage drops without a corresponding coincidental current spike have a cumulative duration less than a predetermined time, assign that identified instance a second classification;
- for each of the identified instances where one or more voltage drops without a corresponding coincidental current spike have a cumulative duration greater than a predetermined time, assign that identified instance a third classification;
- for each of the identified instances where one or more current spikes without a corresponding coincidental voltage drop have a cumulative duration less than a predetermined time, assign that identified instance a fourth classification;
- for each of the identified instances where one or more current spikes without a corresponding coincidental voltage drop have a cumulative duration greater than a predetermined time, assign that identified instance a fifth classification; and
- generate data indicating the assigned classification of each of the identified instances.
31. The plasma generation apparatus of claim 26, wherein the logic circuitry is further configured to calculate a scan energy for each of the identified instances having a first classification.
32. The plasma generation apparatus of claim 26, wherein the plasma generation apparatus is further configured to:
- disable detecting a voltage drop below a predetermined first threshold for a transition hold period after each detection of a voltage drop below the predetermined first threshold; and
- disable detecting a current spike above a predetermined first threshold for a transition hold period after each detection of a current spike above the predetermined first threshold.
33. The plasma generation apparatus of claim 26, wherein the plasma generation apparatus is further configured to adjust the predetermined first voltage threshold during a scanning cycle to track slow changes in the supply voltage.
34. The plasma generation apparatus of claim 26, wherein the duration of voltage drops and the duration of current spikes are measured in clock cycles.
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Type: Grant
Filed: Mar 16, 2007
Date of Patent: Jul 19, 2011
Patent Publication Number: 20080021664
Assignee: Schneider Electric USA, Inc. (Palatine, IL)
Inventors: Alan F. Krauss (Raleigh, NC), Raymond W. Harris (Youngsville, NC), Paul R. Buda (Raleigh, NC)
Primary Examiner: Rodney G McDonald
Application Number: 11/687,615
International Classification: C23C 14/34 (20060101);
